In the various fields of human and veterinary medicine, but also in materials testing, storage films are increasingly being used instead of conventional X-ray films. These storage films contain, in a manner distributed in a thin light-sensitive coating (working coating), centres which pass over into a metastable excited state with a fairly long lifespan when excited by means of X-ray light.
A film of this kind which is exposed by means of irradiating X-ray light contains a latent image in the form of a locally smaller or larger number of excited storage centres, an item of information similar to an X-ray film which has not yet been developed. This item of information is read by scanning the exposed storage film with a laser beam of suitable wavelength (usually in the red range). At the particular point of the storage film which is affected, the laser beam excites storage centres which are in a metastable excited state into a still higher state, from which then relax while emitting fluorescent light. This fluorescent light is measured with a photodetector (generally a photomultiplier).
By recording the output signal of the photodetector in dependence upon the particular instantaneous position of the laser beam used for reading purposes, the latent X-ray image which has been formed by the excited storage centres is converted into an image printed out by electrical signals.
With a view to adequate absorption of X-rays, the density and atomic number of fine particles of a phosphoric material which contains the storage centres must be chosen so as to be sufficiently high. The concentration of the phosphoric particles should also be high.
When reading the X-ray image, the reproduction of the signals or the resolution will be adversely affected if there are additional centres of dispersion, in the form of non-uniformities or mechanical damage, in the light-sensitive coating. Attempts are therefore made to ensure that the light-sensitive coating is, as far as possible, optically isotropic.
Additional centres of dispersion can be imparted to a light-sensitive coating by incorrect handling. These comprise, in particular, scratches or stress marks on the surface of the light-sensitive coating. Since storage films are expensive, it is desirable to avoid such traces of use.
The present invention is directed to addressing these and other issues.